This invention relates to a low-cost X-Y position sensor. More particularly it relates to a so called "sketch pad" that transmits to a digital computer signals indicative of the position of a stylus that is manually moved over the surface of a planar sensing pad. Sensing conductors on the pad are formed in a single layer on the pad and the signals induced in them or transmitted by them are sensed by analog circuitry whose output is converted to digital signals representing the X-Y coordinates of the stylus position.
Electronic sketch pads have long been used to provide inputs to digital computers. They are generally used in an interactive mode in which a cursor on a CRT monitor traces a path corresponding to the path of a stylus moved by an operator over a sensing pad. This arrangement can be used to position the cursor at any desired point on the CRT screen or, alternatively, to draw lines on the screen.
These sketch pads are generally of two types. In one the sensing pad contains two conductor grids, one overlying the other, with the conductors in one grid orthogonal to those in the other grid. To sense the position of the stylus, the wires in one of the grids are first energized sequentially and the signal picked up by the stylus from the wires is sensed to indicate which of the wires is in closest proximity to the stylus. This fixes the position of the stylus along one axis. The wires in the second grid are then sequentially energized to ascertain the stylus position along the other axis. This system is capable of high resolution. However, its cost has significantly limited its applicability.
Another sketch-pad arrangement is described by Dym al. in U.S. Pat. No. 4,087,625. This pad uses a single grid of conductors extending from one edge of the pad across to the opposite edge. The conductors are triangular in shape and are conceptually arranged in complementary pairs. Thus, in each pair one of the conductors has its base along one edge of the pad and its apex at the opposite edge, while the other conductor in the pair has its apex at the first edge and its base at the second edge. To determine the position of a stylus along the coordinate extending between these two edges, one of the conductors in each pair is energized and the other conductor is grounded. The signal picked up by the stylus then varies from a minimum at the edge where the apexes of the energized conductors are located to a maximum at the edge where the bases are located.
This system is less costly than the orthoganal grid system described above since it does not require the relatively complex switching circuitry used in the latter system. However determination of the stylus position along the second coordinate requires the application of either a fairly linear resistive conductor along each of the edges of the pad or the use of a second conductive layer, either of which is a significant element of cost.